1. Field of the Invention
This invention relates to a method for the impartation of blue color to aluminum or an aluminum alloy (hereinafter referred to collectively as "aluminum").
2. Description of the Prior Art
Heretofore, as means to impart blue color to aluminum which has undergone the treatment for formation of an anodic oxide film, several methods centering around the electrolytic coloring process have been known and the method of coloration by immersion using inorganic compounds and other methods using dyes have been known as well.
In these conventional techniques, however, the electrolytic coloring process produces clear blue color only with difficulty. On the other hand, the method of coloration by immersion using an inorganic compound entails the disadvantage that the coloring substance used therein does not easily permeate to the depth of micropores of the anodic film which in turn results in the discoloration of the treated aluminum and the contamination of the bath in the aftertreatment. It is also difficult to obtain within a short span of time the impartation of blue color of desired density to the anodic oxide film produced by the conventional method. By the method of dyeing, it is difficult to attain durable coloration of the anodic oxide film of medium thickness (about 10 .mu.m).
As measures to solve these problems, Japanese Patent Publication No. SHO 52-5010 proposes a method for effecting the coloration of aluminum by subjecting aluminum to anodic oxidation in an aqueous phosphoric acid solution and immersing the treated aluminum in a bath of fine dispersion of an aqueous pigment thereby inducing adsorption of the pigment to a porous anodic oxide film formed on the surface of aluminum or a method which further coats the colored aluminum obtained as described above with a thermosetting resin, Japanese Patent Publication No. SHO 51-35177 proposes a method for attaining the coloration of aluminum by immersing aluminum which has undergone anodic oxidation in a nonionic or nonionic-cationic bath of fine dispersion of an aqueous organic pigment and passing a direct current or an alternating current through the aluminum in the bath thereby inducing adsorption of the pigment to a porous anodic oxide film formed on the surface of aluminum or a method which further coats the colored aluminum obtained as described above with a thermosetting resin. These patent publications disclose working examples which attained coloration of an anodic oxide film of aluminum in blue.
These patent publications teach that the fine dispersion of the pigment having a particle size falling in the neighborhood of 1 .mu.m (1,000 nm), preferably not exceeding 0.5 .mu.m (500 nm) is used. The anodic oxide film for which the fine dispersion of the pigment is to be used generally has a pore diameter of not more than 50 nm. Since most pigment particles are larger than the pore diameter, therefore, the coloration of aluminum occurs in such a manner that the pigment is deposited by adsorption in a layer in the mouths of pores in the anodic oxide film and on the surface of the oxide film. The colored and pore-sealed aluminum of such a method, therefore, poses a problem of poor fastness of the imparted color to the impact of abrasion and consequent ready release of the pigment and, moreover, entails the disadvantage that fast coloration is not obtained unless the colored oxide film is coated with resin as disclosed in the patent publications mentioned above.